1. Field of the Invention
This invention relates to buttress thread fasteners, and more particularly, to a buttress thread with a reduced thread root stress for fatigue resistance.
2. Description of the Prior Art
Fatigue resistance on fastener threads is important when the fasteners are subjected to cyclic loading. Such cyclic loading can occur in many applications, such as pressure vessels and in pumps and compressors in which the pressure varies, thus also varying the loads on the fasteners used therein. In most instances, normal bolting is satisfactory, but there are some applications in which special thread forms are desirable.
One such case is the thread on cover retainers used on plunger pumps such as the Halliburton Model HT-400. The cover retainers are used to retain valves and rupture discs in the fluid end of the pump, and the diameter of the thread is relatively large compared to the length of thread engagement. These cover retainers are used to actually clamp parts in place and are not limited to simply join two members together. Thread forms, such as square and Acme threads frequently used for power transmission, have been applied to these pump applications. However, such threads have relatively sharp corners and thus are not particularly well adapted for fatigue resistance.
One thread form which has been developed for heavy loading, and is good in clamping applications, is the American Standard buttress thread--American National Standards Institute (ANSI B1.9. This thread has a 7.degree. load flank angle and a 45.degree. relief flank angle and is quite strong. However, root stresses in this thread are actually higher than the normal Acme thread. Therefore, the ANSI B1.9 thread is also not well adapted for fatigue resistance.
The present invention which is a modified buttress thread having a relatively small thread height with a relatively large root radius provides the strength desired while greatly improving the fatigue life. In particular, for plunger pump applications, this improvement is very important, and the thread still retains its ability to clamp the corresponding components in place.